Research On Origin And Sedimentary Environment Of Lower Cambrian Of Hetang Formation Cherts In Northwestern Zheiiang,China

Chert series occur in The Lower Cambrian of Hetang Formation in Northwestern Zhejiang Province,which are rich in elements such as Ni,Mo,V,Cu,U,Ba,Pt,Pd,Au and Ag and characterized by abundant organic matter and well hydrocarbon generation potential,distributed in South China and is an important metallogenic layer and a good source rock.Due to the divergence of the origin,sedimentary environment and organic accmulation of the chert series,this paper focus on Hetang cherts in the Shimen section of Lin'an,Diben and Daqian section of Kaihua,Fangcun section of Changshan,Wujialing section of Jiangshan,Langwudi and Sheng zhaowu section of Zhuji in the Northwest Zhejiang Province,and based on field measured section,microscopic examination,primary and trace geochemistry,total organic carbon content,total rock oxygen isotope,organic carbon isotope,and trace elements in pyrite,the sources of cherts and organic matter in the Hetang Formation were identified,the genesis,sedimentary environment and sedimentary evolution of cherts were clarified.The research results provided an important reference for the Lower Cambrian mine exploration and shale gas exploration in South China and have important scientific significance for the analysis of the tectonic evolution of the Caledonian sedimentary basin in the northwestern Zhejiang and the Caledonian sea-land pattern and tectonic process in the Lower Yangtze region.The main results of this study are as follows:?1?The cherts of the Hetang in the Northwestern Zhejiang were mostly gray-black to black and thin produced with high carbon content.The distribute of cherts are along the Jiangshao fault gradually thickened from east to west.The microscope examination shows that the minerals of cherts were mainly quartz,organic matter,mica and pyrite,and siliceous sponge and radiolaria could be found.?2?The content of SiO₂ in the cherts of the Hetang Formation in the Northwestern Zhejiang Province was medium?>70%?,and the components with the content above 1%were TFe₂O₃ and Al₂O₃,and the other primary chemical components were very low?generally<1%?.The values of the constant elements Fe/Ti,?Fe+Mn?/Ti and Al/?Al+Fe+Mn?ratio were 71.25,71.28 and 0.41 in average,respectively,and value of the trace elements?La/Ce?N,?La/Yb?N and U/Th ratio were 1.27,1.01,7.61 in average,respectively.The total amount of rare earth was low,the ratio of ELREE/E HREE was 0.74,and the heavy rare earth was enriched.Generally,?Ce was negatively abnormal,8Eu was positively abnormal,and the PAAS standardized rare earth partitioning pattern was similar to typical hot water sediments.The ?¹8O isotope value was distributed in the range of hot thermal quartz and mosaic quartz.The average temperature of the siliceous rock formed by the equilibrium fractionation equation was 79.19 ?,these geochemical fetures suggest hydrothermal influce upon the process of diagenesis of the cherts.?3?Through the analysis of the constant and trace R-factors of cherts,3 factors with diagenetic indications were extracted from the primary elements:terrestrial factors,hydrothermal factors and autotrophic factors.Among them,terrestrial factors?Al₂O₃,TiO2,SiO₂,K₂O,Na₂O?,hydrothermal factors?MnO,CaO,,P₂O₅,Fe₂O₃,?MgO??,autotrophic factors?Fe₂O₃,SiO₂,,P₂O₅?.The factor analysis of trace elements were extracted F,?Be,Sc,Ga,Rb,Zr,Nb,Cs,Hf,Ta,Th and other large ion lithophile elements and high field strength elements?,and F₂?V,Cr,Ni,Zn,Y,Ba,Pb,U,and other pro-iron elements?,and Fi,F₂ variance contribution rate was equivalent.The main factor analysis of the primary traces indicated that the composition of cherts of the Hetang Formation was affected by both terrestrial and hydrothermal fluids,but the correlation between SiO₂ and terrestrial factors was more obvious,indicating that the silicon of the Hetang Formation cherts was mainly from the terrestrial sources.In addition,the ratios of Y/Ho and Ti/V of the whole rock were 34.12 and 14.68,respectively,which was much larger than the value of the traditional hydrothermal cherts,and the ratio of the trace element Co/Ni of the layered pyrite in the cherts was 0.37.The Y/Ho ratio was 23.5,which was close to the crustal characteristics of the Eastern China continent,indicating that the source of cherts was controlled by terrestrial materials.?4?They are suggested that the Hetang Formation developed on the passive continental margin by the tectonic discriminant of both primary and trace elements of the whole rock.And combined with the entire Caledonian period in China when there was no ocean inside the Yangtze block and between the Huaxia and Yangtze blocks,this paper believed that the Hetang Formation developed in the rift basin.According to the exposed location of the Hetang Formation,the Langwudi section in Zhuji,Diben section in Kaihua,and Fangcun section in Changshan,which they were in common near the Xiaoshan-Qiuchuan Fault and the Majin-Wuzhen Fault were greatly affected by the hydrothermal fluid;the Daqian section in Kaihua,Shimen section in Lin'an and Shengzhaowu section in Zhuji,it could be found that the sections far from the deep fault were less affected by hydrothermal fluids,which indicating that the deep faults in the rift basin were the hydrothermal source channels in the sedimentary stage.?5?Through the vertical evolution characteristics analysis of elements such as V/?V+Ni?,V/Cr,Ni/Co,the bottom of the Hetang Formation was most affected by the hydrothermal fluid,and the redox evolution indicated a general trend of anaerobic reduction-hypoxia reduction.The ?13Corg showed a steady upward trend from the bottom to the top,indicating that the water gradually became shallower.?6?The organic carbon content?TOC?of the cherts of the Hetang Formation in the Northwestern Zhejiang was higher,with an average of 3.75%.The organic carbon isotope ?¹3Corg and type I kerogen indicated that the organic matter of the Hetang Formation in the Northwestern Zhejiang was mainly derived from marine algae and rich aquatic plants.The correlation between organic carbon content and redox changes in water bodies indicated that the continental stagnation stagnation basins in the Northwestern Zhejiang and the faults which can provide channel to hydrothermal,can contrubute to anoxia environment,which is the root cause of the organic accmulation of the chert of the Hetang Formation in the Northwestern Zhejiang.